A permanent magnetic synchronous motor (PMSM) can be controlled with a location sensor or without a location sensor. For a control method with the location sensor, a magnetic field location of the synchronous motor is detected by the location sensor, whereas for a control method without the location sensor, a rotor location of the synchronous motor is estimated by a location estimation module. An out-of-step of the synchronous motor is defined with respect to a synchronization, i.e., a rotating magnetic field of a stator does not rotate synchronously with the magnetic field of a rotor magnet. In the PMSM system, reasons such as high system load or reduced motor performance will lead to the out-of-step of the synchronous motor. When the motor is out of step, the rotating speed of the motor is generally uncontrollable, which will cause serious losses and safety risks. Thus, there is a need to check the out-of-step of the synchronous motor precisely and timely.